In known fuel injector designs, a nozzle control valve is provided to control movement of a fuel injector valve needle relative to a seating and, thus, to control the delivery of fuel from the injector. A so-called Electronic Unit Injector (EUI) is an example of such an injector. An Electronic Unit Injector includes a dedicated pump having a cam-driven plunger for raising fuel pressure within a pump chamber, and an injection nozzle through which fuel is injected into an associated engine cylinder. A spill valve is operable to control the pressure of the fuel within the pump chamber. When the spill valve is in an open position, the pump chamber communicates with a low pressure fuel reservoir so that fuel pressure within the pump chamber is not substantially affected by movement of the plunger and fuel is simply drawn into and displaced from the pump chamber as the plunger reciprocates. Closure of the spill valve causes pressure in the pump chamber to rise as the plunger is driven to reduce the volume of the pump chamber. Each Electronic Unit Injector has an electronically controlled nozzle control valve that is arranged to control the timing of commencement and termination of the injection of fuel into an associated engine cylinder. Typically, the engine is provided with a plurality of Electronic Unit Injectors, one for each cylinder of the engine.
Although the use of a nozzle control valve in an Electronic Unit Injector provides a capability for controlling the injection timing, and such units are capable of achieving high injection pressures, both injection pressure and injection timing are limited to some extent by the nature of the associated cam drive.
In common rail fuel injection systems, a single pump is arranged to charge an accumulator volume, or common rail, with high pressure fuel for supply to a plurality of injectors of the fuel system. As in an Electronic Unit Injector, the timing of injection is controlled by means of a nozzle control valve associated with each injector. One advantage of the common rail system is that the timing of injection of fuel at high pressure is not dependent upon a cam drive, and so fast and accurate control of the timing of injection can be achieved with the nozzle control valves. However, achieving very high injection pressure within a common rail system is problematic and the high levels to which fuel must be pressurised can cause high stresses within the pump and within the rail. The rail must therefore be provided with a relatively thick wall for pressure containment, making it heavy and bulky. Parasitic fuel losses can also be high.
It has been recognised that significant improvements in combustion quality and efficiency may be achieved by rapidly varying the injection pressure level and injection rate within an injection event. Such variations in the injection characteristics can be difficult to achieve rapidly with both Electronic Unit Injector systems and common rail systems, and the efficiency of both types of system is limited. For example, in a common rail system designed to achieve injection at a high rail pressure, it is also possible to achieve a lower injection pressure by relieving some of the high pressure fuel to a low pressure reservoir. This, however, is an inefficient use of pumping energy.
It is a feature of common rail systems that in order to terminate injection it is usually necessary to apply a high hydraulic force to the back end of the injector valve needle, and this is achieved through operation of the nozzle control valve. It has been found, however, that this results in a disruption of the fuel spray formation into the engine cylinder, and produces an unnecessary degree of smoke.
The present invention is aimed at one or more of the problems set forth above.